Certain polymeric films have been found to be capable of exhibiting pyroelectric properties, i.e., capable of reacting, in terms of electrical charge polarization, to a change in temperature of the material, and piezoelectric properties, i.e., capable of reacting in the same manner to changes in strain of said film. These films are typically poled, which essentially involves the application of an intense electric field gradient thereto. In essence, the film after poling retains an electrical polarization and becomes substantially stable relative thereto unless changes in strain (piezoelectric) or temperature (pyroelectric) occur. One such film which has found exceptional use in this area is polyvinylidene fluoride.
Polyvinylidene fluoride, when manufactured in film form, such as by an extruder, etc., has a crystalline structure which is in essence of the alpha type. It has been found, however, that the alpha crystalline film, when poled, does not provide a film displaying sufficient pyroelectric or piezoelectric properties for practical utility. Therefore, the crystallinity of the film is typically changed to the beta type, which is susceptible to successful poling to render the film sufficiently pyroelectric and piezoelectric. Typically, this change in the crystalline structure from alpha to beta is undertaken by biaxial or uniaxial orientation, i.e., stretching the film in a heated state and retaining same in the stretched state while the film is allowed to cool. The resultant film exhibits the beta crystallinity which can, as aforementioned, be successfully poled to render same pyroelectric and piezoelectric.
We have now found that by using a blend of polymers, processing of same to a film or a coating on an article can be undertaken without the necessity of mechanical orientation of the film to the beta crystallinity, and yet a film or coating displaying pyroelectric and isotropic piezoelectric characteristics can be obtained